1. Field of the Invention
The invention relates to an arrangement for plural variable resistors on a common actuation shaft.
2. The Prior Art
In order to enable reasonable and low-cost production of the numerous variations demanded by the user of one type of variable resistor, variable resistors are assembled in the so-called modular fashion. In this way, it is more readily possible to fulfil the requirements of the user for single-, tandem-, duplotandem-, duotandem-, doubletandem-, etc., variable resistors, with specified requirements as to shaft configurations and resistance values, with a resistor whose major components are the same for all variations.
A variable resistor is already known which is modularly constructed such that several modules of the same kind are couplable with one another. Each module consists of a pot-shaped housing having a front and a back surface. A spring carrier is rotatably mounted in the housing with the aid of a shaft which extends through a central opening in the housing. A contact spring is mounted on the spring carrier, the contacts of the spring resiliently resting against a resistance course as well as against a collection course. The resistance course is mounted on a carrier plate which forms the end of the pot-shaped housing. The collector is developed as a planar collector ring with a central opening, the ring being arranged in the bottom of the pot-shaped housing and having a central opening through which the shaft extends. Thus, the contacts of the contact spring are arranged on different sides of the spring carrier. The housing itself is connected, by a locating plate, with an externally threaded hub-flange.
To obtain better protection against unintentional adjustment of the variable resistor and to obtain precisely defined resistance values, for example, for use with volume control systems, conventional variable resistors are additionally provided with a click-stop device. Ordinarily, an odd number of stops is chosen in order to make possible a defined center position, for example, when used for audio adjustment. If precision adjustment is required, as many as 41 stop-positions may be provided. With simpler apparatus, a single center-position stop may suffice.
A variable resistor having a click-stop device, consisting of a hub-flange on which a carrier plate is mounted for carrying the resistance course and the collector course, is already known. An actuation shaft extends through the hub-flange; on one end of the actuation shaft a spring carrier is mounted. On the side of the spring carrier turned towards the carrier plate, a contact spring is mounted with its contacts lying resiliently against the resistance and collector courses in order to produce an electrical connection between the two courses. A row of dome-like elevations are provided along a circumference at a predetermined radius from the axis of said spring carrier, and said elevations cooperate with two diametrically-opposed projecting noses of a stop-spring. This stop-spring is arranged on a pot-shaped housing which surrounds the entire variable resistor and which is mounted on the carrier plate. It is a disadvantage that this click-stop device is not readily transferable to a variable resistor of modular design.
With known variable resistors, each spring carrier is ordinarily turned independently by the actuation shaft. Because the coupling between the spring carrier and the shaft is a loose one and the construction parts participating in the transmission of the turning movement are subject to the usual functional tolerances, a relatively low level of synchronous precision results for tandem resistors of this type. By the term synchronous is meant the electrical uniformity of the individual resistance values realized within a predetermined actuation zone. The measurement is done by a voltage comparison in which the synchronous tolerance is calculated in dB from the common logarithm of the ratio between the two voltages tapped, multiplied by a factor of 20. With known variable resistor arrangements, a synchronism of only 3 dB can be attained. For high-quality apparatus, however, synchronism conditions of 1 dB and less are required.
A tandem resistor is already known in which the actuation shaft between the mounting positions of the two spring carriers has a reduced cross-section. This cross-section reduction is obtained by an indentation of the shaft, or by using two surface millings symmetrically displaced 180.degree. apart, or by means of a transverse bore through the shaft. Torsional stress on the actuation shaft causes a deformation at the portion with reduced cross-section, such that the spring carriers mounted on the actuation shaft can be turned relative to one another. In this manner, a synchronisation correction is made possible. To facilitate the twisting, the end of the actuation shaft is provided with a screw driver slot. With this arrangement it is disadvantageous that no possibility of having a detent position exists.